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United States Department of Agriculture

Agricultural Research Service

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Location: Emerging Pests and Pathogens Research

2013 Annual Report

1a. Objectives (from AD-416):
Determine the effect of habitat and repeated foliar damage on the growth, reproduction, and survival of black and pale swallow-wort plants, and document the survival and longevity of swallow-wort seed in the soil seed bank.

1b. Approach (from AD-416):
Established seedlings of pale and black swallow-wort will be tagged within three habitat types and annually censused for survival, growth and time to first seed production. Mature plants of pale and black swallow-wort will be planted into a common garden and receive damage treatments consisting of clipping, 50% or 100% defoliation at different frequencies over six years. Various growth and reproductive parameters will be measured. Subsets of plants will be harvested during that time to assess changes in plant biomass. Seed of pale and black swallow-wort will be placed at various depths into pots sunken into the ground and assessed for germination and survival rates over four years.

3. Progress Report:
Various studies of pale and black swallow-wort biology and ecology are being conducted to help determine what factors affect their individual and population growth and spread, what life stages should be targeted for disruption through biological control, and how effective other control measures may be. Data collection continues for a field study established in fall 2007 of the effect of habitat (field, forest edge, and forest interior) on juvenile swallow-wort survival, growth, and time to first flower and seed production. After the fifth season of growth (2012), survival of black swallow-wort (black SW, 24%) was greater than pale swallow-wort (pale SW, 3%). Juvenile black SW plants remained approximately twice as tall as pale SW plants. Three new black SW plants growing in the open field flowered in 2012 with two producing seeds. To date only black SW has reproduced after 4-5 years of growth. Some plants flower earlier but do not produce any seed. Low (and decreasing) survival is attributed to both a dense cover of resident vegetation in the old field as well as wetter conditions than swallow-wort tolerates. A long-term field experiment examining the effects of different types and frequencies of foliage removal on pale and black SW performance continues. The eight damage treatments are: 50% defoliation (all new leaves cut in half widthwise plus stem tips cut) or 100% defoliation once (July) or twice (June & July) each season, clipping stems 8 cm above soil level once (June), twice (June & July) or four times (May, June, July, Aug.) each season, and undamaged control plants. Plants generally continue to increase in size regardless of the damage they receive, e.g. root dry mass averaged 65 g in 2012 versus 19 g in 2009. Only plants clipped four times each season have not increased their root mass since 2009. A similar pattern was observed for shoot mass and seed production. No plant deaths have occurred as a result of damage treatments. An article detailing results of a two-year (2008-2009) herbicide and mowing study that we have previously reported on for pale SW in an old field and adjacent forest understory site near Ithaca, NY is expected to be published in early September 2013 in the journal Invasive Plant Science and Management. Several treatment combinations were shown to effectively control pale SW in both habitats. These may be valuable short-term options for swallow-wort management while potential biological control candidates are evaluated. A multi-year seed bank study was initiated in fall 2011 to assess the longevity of seed of pale SW and black SW at four different burial depths (0, 1, 5, and 10 cm). The experiment was conducted in pots buried with the rim nearly flush with the surrounding soil and filled with a locally collected soil. One hundred swallow-wort seeds were sown in each pot. During the 2012 growing season, pale SW had relatively poor emergence at the 0 cm (9%), 5 cm (6%), and 10 cm (4%) sowing depths, while at 1 cm 38% of pale SW seeds emerged. The larger seeded black SW was more successful, with two-thirds of all sown seeds emerging at the 1 cm (67%) and 5 cm (66%) depths and 26% emerging at 10 cm. Only 6% of the surface-sown black SW emerged. In both species most germination occurred during the spring, with 79% of pale SW and 86% of black SW seedlings emerging before June 21. The viability of recovered seeds harvested in the fall of 2012 is currently being evaluated. However, no seeds that appeared to have an embryo (i.e. ‘filled’ seed) were recovered from any of the treatments except for black SW seeds sown at the 0 cm depth. A key finding from this first sampling year is that the two swallow-worts can germinate and emerge from soil depths of 10 cm which was not expected and is new and important information. A greenhouse experiment was conducted to supplement a previous study of the photosynthetic abilities of pale and black SW. We examined the plasticity of these two species based on photosynthetic performance in response to varying light environment. In particular, this experiment measured the ability of black SW to photosynthesize under low light conditions, similar to that found in heavily-shaded forest understories, in which black SW is not normally observed. Although maximum photosynthetic rates between the two species were comparable, the high light compensation levels measured for black SW plants indicate that this species may be more adapted to high light environments than pale SW. Pale SW did not exhibit greater maximum photosynthetic rates under constant shade, but other parameters measured suggest a higher degree of shade adaptation than black SW. Both species showed a plastic response to the changing light conditions with black SW adjusting to a greater extent than was expected of a sun-adapted species. Although pale SW occurs in a wide range of light environments, its photosynthetic performance was comparable to that of black SW despite the latter being generally absent in heavily shaded habitats such as forest understories. These data will be added to a manuscript currently being revised and expected to be submitted later this fall.

4. Accomplishments

Last Modified: 06/22/2017
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